There have been two different times I found crawlies on some of my chickens over the past two years. Both times, I dusted all of them, whether they had them or not, with DE, as well as the floors of the coops, their nesting boxes, and their favorite dust bathing spots. Both times, the crawlies were gone in a couple days. I checked them frequently for a couple weeks after just to make sure.
Now, I sprinkle DE in their coop floors since I use sand as the coop bedding and dust each bird at the start of winter and again at the start of summer for prevention. I just now checked all of the chickens since I sold my Australorps, and I didn't see a single crawly on any of them.
I have never had a full blown infestation, so I can't attest to how well DE would work in that situation, but the two times I had a small problem, it fixed it very quickly. I try to stay as natural as possible, so DE is my first choice. They consume a little, too, since they eat off the sand in their coop, and that is said to be good for internal parasites and cleansing, too, so even better.
Be careful with DE, though, for two things. You don't want to breathe it in, and you don't want them to breathe a bunch in either. Also, don't use it around gardens or flowers. It will kill bees just like it does other bugs, and we need bees! I won't use it for pest control of the garden for that reason.